The present invention relates to a multi-beam optical system such as to be employed in a multi-beam laser plotter for forming a plurality of beam spots on an object surface to be exposed, and more particularly to a multi-beam optical system wherein a diffractive beam-dividing element is used to divide a light beam emitted from a single light source into a plurality of separate light beams.
A multi-beam optical system of the above-mentioned type has conventionally employed a prism-type beam splitter as a beam-dividing element, which comprises a plurality of prism blocks cemented to one another. The cemented faces of the prism blocks are provided with multi-layer coatings having the desired reflecting properties, respectively.
With employing aphorism-type beam splitter, however, as each one of the multi-layer coatings could divide one incident beam only into two separate output beams, the number of the prism blocks corresponding to the required number of the separate beams must be cemented to one another, and when cementing one block to another block, a positional error between two blocks unavoidably arise if it is very small. Accordingly, when the large number of the separate beams are required, the deviations of the beam spots on the object surface tend to become large due to accumulation of positional errors between the cemented prism blocks.
Recently, a diffractive beam-dividing element has become used in place of a prism-type beam splitter. Since the diffractive beam-dividing element is made of a single block that is not cemented, it does not generate any positional error even when the large number of the separate beams are required.
With employing the diffractive beam-dividing element, however, since the diffraction angle of a light beam varies depending upon the wavelength thereof, the same order diffracted beam may form a plurality of beam spots indifferent positions on the surface to be exposed, in case a light source emits a light beam having a plurality of peak wavelengths.
For example, an argon laser, which is used as a light source of a laser photo plotter or the like, has a plurality of peak wavelengths in the ultraviolet and visible regions. Therefore, in order to avoid the above defects, it has been required to use a filter for passing a beam component of a selected peak wavelength. Thus, the beam components of peak wavelengths other than the selected peak wavelength are cut off by the filter, which results in low energy efficiency.
Further, even if a beam emitted from a light source has a single peak wavelength, in case a peak wavelength of a beam actually emitted from a light source fluctuates or varies, a beam spot pitch on a surface to be exposed is changed. The same can be said even in case a filter is used as explained hereinabove.
It Is therefore an object of the present invention to provide an improved multi-beam optical system capable of avoiding the defects caused by the wavelength dependency of a diffractive beam-dividing element employed therein.
For the above object, according to the present invention, there is provided an improved multi-beam optical system, which includes:
a light source that emits a light beam having a plurality of peak wavelengths;
a diffractive beam dividing element that divides the light beam emitted from the light source into a plurality of light beams exiting at the different diffraction angles, respectively; and
a compensating optical system whose lateral magnification is inversely proportional to the wavelength of the light beam to compensate deviation of the diffraction angle of the diffracted light beams caused due to wavelength dependence of the diffractive beam-dividing element.
With this construction, the compensating optical system reduces the deviation of the diffraction angles of the light beams that have different peak wavelengths and are diffracted by the diffractive beam-dividing element, which allows the compensating optical system to align the same order diffraction beams diffracted by the different diffraction angles at the same position. Since the chromatic aberration caused by the diffractive beam-dividing element is too large to be corrected by a refractive lens system, the compensating optical system includes a diffractive lens component.
Further, the compensating optical system preferably includes:
a first group that has a positive power with a positive chromatic dispersion as a whole, the first group including a diffractive lens component having a negative power; and
a second group that has a positive power with a negative chromatic dispersion as a whole, the second group including a diffractive lens component having a positive power.
In the specification, the positive chromatic dispersion is defined as such a wavelength dependence that an equivalent refractive index increases as wavelength decreases.
When the light beam incident on the diffractive beam-dividing element is a parallel beam, it is preferable to locate a converging element between the diffractive beam-dividing element and the compensating optical system to converge the parallel beams exiting from the diffractive beam-dividing element. Since the converging element is preferably free from the chromatic aberration, it may be a concave mirror or a refractive lens with a diffractive lens component that corrects the chromatic aberration.
Still further, each of the diffractive lens components of the first and second groups may be provided on a lens surface of the positive refractive lens or a mirror surface of a concave mirror. When the diffractive lens component is provided on the mirror surfaces, the diffractive lens component only generates the chromatic dispersion which has a linear relationship with the wavelength of the light beam, which is suitable to cancel the linear chromatic dispersion caused by the diffraction beam-dividing element.
Specifically, it is preferable that the first and second groups satisfy the following conditions (1) and (2).                                           λ            B                                λ            A                           less than                                                             f                1                            ⁡                              (                                  λ                  A                                )                                      +                                          f                2                            ⁡                              (                                  λ                  A                                )                                                                                        f                1                            ⁡                              (                                  λ                  B                                )                                      +                                          f                2                            ⁡                              (                                  λ                  B                                )                                                     less than                               λ            A                                λ            B                                              (        1        )                                                                    λ              A                                      λ              B                                       less than                                                             f                1                            ⁡                              (                                  λ                  A                                )                                                                    f                2                            ⁡                              (                                  λ                  A                                )                                              xc3x97                                                    f                2                            ⁡                              (                                  λ                  B                                )                                                                    f                1                            ⁡                              (                                  λ                  B                                )                                                     less than                               (                                                            λ                  A                                                  λ                  B                                                      )                    3                                    (        2        )            
where,
xcexA, xcexB: peak wavelengths to be used among the plurality of peak wavelengths of the light source (xcexA greater than xcexB),
f1(xcexA): the focal length of the first group at the wavelength xcexA,
f1(xcexB): the focal length of the first group at the wavelength xcexB,
f2(xcexA) : the focal length of the second group at the wavelength xcexA, and
f2(xcexB): the focal length of the second group at the wavelength xcexB.
When the multi-beam optical system is applied to a scanning optical system, the compensating optical system is preferably located between the diffractive beam-dividing element and a deflector. Further preferably, the compensating optical system may be located between the diffraction beam-dividing element and a multi-channel modulator that independently modulates each of the plurality of light beams exiting from the compensating optical system.
When the multi-beam optical system is used as a component of the scanning optical system, the compensating optical system functions as a relay lens that relays the diffractive beam-dividing element and the deflector such as a polygonal mirror. When the lateral magnification of the relay lens is inversely proportional to the wavelength of the light beam, the composite focal length of the optical system at the imaging side, i.e., in and after the diffractive beam-dividing element is inversely proportional to the wavelength, which compensates deviation of the diffraction angle of the diffracted light beams caused due to wavelength dependence of the diffraction beam-dividing element. Further, when the relay lens is an a focal lens, the angular magnification thereof is preferably proportional to the wavelength.